The `Tree-Kangaroos Come First' hypothesis

One of my favourite groups of marsupials are the wonderful tree-kangaroos. There are presently ten recognised tree-kangaroo species; they occur exclusively on New Guinea, Umboi, New Britain and north-east Queensland (and it’s generally thought that they were introduced to Umboi by humans).

Tree-kangaroos first became known to Europeans in 1826 when crew of the Dutch sailing ship The Triton collected four ‘wangoerie’ specimens (kept as pets by local people) from the north coast of New Guinea. Salomon Müller published the name Dendrolagus for these animals in 1842. It means ‘tree hare’ - the ‘hare’ part of the name apparently being a reference to the game-like quality of tree-kangaroo meat.

Some tree-kangaroo species have only been discovered very recently. Two new taxa (Seri’s tree-kangaroo D. stellarum and the Tenkile D. scottae) were named in 1990. And in the same year, wildlife photographer Gerald Cubitt encountered a Dani tribesman in the Sudirman Range, Papua, who had recently captured individuals of a distinctive, mostly black tree-kangaroo. This animal was known to locals as the Dingiso and it seemed to be far more terrestrial than other tree-kangaroos. It was formally named as a new species (D. mbaiso) in 1993. The Golden-mantled tree-kangaroo D. goodfellowi pulcherrimus was also named in 1993. If you’re interested in knowing more about the history and discovery of tree-kangaroos, be sure to track down Tim Flannery et al.’s Tree Kangaroos: A Curious Natural History (Flannery et al. 1996) and Roger Martin’s Tree-Kangaroos of Australia and New Guinea (Martin 2005).

Those boring stereotypes and crass mischaracterisations

As is the case with so many groups of animals, the same set of facts tend to be trotted out about tree-kangaroos whenever you read about them. One is that they are clumsy and overall poor at climbing. This really isn’t a fair criticism and I wonder if it’s laced with the implication that marsupial tree-climbers are inferior when compared with the wonderful squirrels and monkeys with their Magic Placenta Powers, combined with the notion that tree-climbing kangaroos are just absurd and clearly some sort of evolutionary joke. In fact, studies of climbing behaviour in tree-kangaroos show that they’re proficient at rapid vertical scaling of tree trunks (so long as there are enough boles, patches of rough bark and climbing plants to provide purchase); they’re also good at balancing on and traversing narrow horizontal supports, and they’re agile leapers in the tree-top environment.

I remember getting the impression – actually from the literature on bird origins – that tree-kangaroos are scarcely modified for their lifestyle; in other words that they lack obvious arboreal specialisations. This isn’t in the least bit accurate (Dececchi & Larsson 2011).

In the upper arm, a large rotator cuff is present (Flannery et al. 1996): this is the name given to a linked set of muscles that allow a particularly wide range of forelimb movement. The long, slender, sometimes tasselled, tails of tree-kangaroos are used in maintaining balance while climbing and are rather different from the proportionally shorter, more muscular, more ‘springy’ tails seen in many other macropods.

Tree-kangaroo muscle mass is about one-third less that of terrestrial macropods (Grand 1990) [for the record, “macropod” = all members of the kangaroo radiation, from rat-kangaroos and potoroos to the many wallaby and true kangaroo lineages]. This could be interpreted as a weight-saving adaptation, but it’s also likely due to their proportionally short hindlegs and reduction in the muscles required for perpetual terrestrial bounding. Tree-kangaroo fur is peculiar in growing in whorls, supposedly an adaptation for rain-shedding (the hair on the neck grows towards the head, rather than toward the shoulders). In some tree-kangaroo species, the whorls are on the shoulders, in others they’re in the middle of the back, and in others they’re located at the base of the tail.

Tree-kangaroo limb joints are obviously extremely able to absorb massive amounts of shock and compression (I have the idea that this is because they possess proportionally enormous cartilage pads and capsules, but I can’t find out where I ‘learnt’ this). I say this because tree-kangaroos engage in a pretty radical bit of defensive behaviour. When threatened while in the tree-tops they leap to the ground – even if it’s more than 15 m away. Martin (2005) noted that, while this might work fine when confronted by pythons, big owls, eagles, or even thylacoleonids (tree-kangaroos would almost certainly have faced predation from this group in the recent past), it isn’t so effective when the hunters are humans and their dogs. Obviously, humans can flush a tree-kangaroo from an arboreal refuge, making it an easy target after pursuit on the ground.

Once on the ground, tree-kangaroos hop bipedally in conventional macropod fashion, though usually with the long tail held up and over the body. They frequently get hit by cars, and I have to note the interesting suggestion that some sightings of ‘Queensland tigers’ might be fleeting glimpses of terrestrial tree-kangaroos, seen disappearing into bush (Martin 2005, p. 30). Also worthy of mention is the fact that tree-kangaroos are unique among macropods in being able to walk by moving their hindlimbs independently of one another (Windsor & Dagg 1971). Other macropods can’t do this: they can only move their hindlimbs synchronously and thus cannot ‘walk’ as we normally understand the term.

Anyway, Martin (2005, p. 98) refers to one particularly impressive vertical leap made by a Bennett’s tree-kangaroo during the night. He was able to return the next day and measure the height of the branch the kangaroo had leapt from. It was 22 m off the ground, yet the animal had bounded off, apparently without injury. Procter-Gray & Ganslosser (1986) studied the different forms of movement practised by Lumholtz’s tree-kangaroo D. lumholtzi and used the term ‘crash’ for these impressive vertical leaps. They also stated that, after performing a ‘crash’, an individual “landed on its feet or all fours, and was immediately able to begin bipedal leaping on the ground. Never did a tree-kangaroo appear injured or stunned after a crash” (p. 347). On landing on the ground, the animals produce (unsurprisingly) a loud thud. Jared Diamond once said “The loud thuds of falling tree-kangaroos as they hit the ground were among the distinctive sounds that I heard daily in the Foja Mountains” (Diamond 1997, p. 692).

We should note that tree-kangaroos don’t always make incredible leaps like this when threatened, since individuals of some species are also reported to freeze when confronted by predators and thus hope to avoid attention by crypsis.

Kangaroos belong to a large group of Australasian marsupials called diprotodontians, and ancestral diprotodontians were - based on the anatomy and ecology of early fossil diprotodontians and of the distribution of climbing habits in living species - almost definitely able tree-climbers. Evidently, early members of the kangaroo clade became specialised for life on the ground. In the standard scenario, one kangaroo lineage then became specialised anew for scansorial and eventually arboreal life, giving rise to the tree-kangaroos. And then at least one tree-kangaroo lineage – the one that led to the Dingiso – seemingly became terrestrial again… or, at least, more terrestrial than other tree-kangaroos, anyway.

Authors have typically been rather vague with respect to where tree-kangaroos might fit within the macropod radiation. Taxonomic lists, dendrograms and classification sequences have generally placed tree-kangaroos (sometimes as their own ‘tribe’, Dendrolagini, within the advanced macropod ‘subfamily’ Macropodinae) outside the group that includes true kangaroos and wallabies, and sometimes close to the New Guinea forest wallabies (e.g., Raven & Gregory 1946, Nowak 1999, Kear & Cooke 2001). The implication from such a placement is that tree-kangaroos evolved from an early, primitive member of Macropodinae. Because members of this clade are known from the Middle Miocene, tree-kangaroos must have diverged by this time or earlier.

‘Tree-Kangaroos Come First’?

But tree-kangaroos are really unusual compared to other macropods. They seem weirdly, well, ‘primitive’.

For one thing, their short-leggedness is obvious. And their legs aren’t just short: the proportions of their legs (talking hindlegs here) are odd for a macropod in that their shin and thigh bones are very similar in length (Kear et al. 2008). Similar proportions are seen elsewhere in climbing phalangers like the Common brushtail possum Trichosurus vulpecula. For comparison, the tibia is nearly twice as long as the femur in big, specialised saltatorial macropods like Macropus and Sthenurus.

Tree-kangaroos are also unusual compared to other macropods in lacking extensive contact between the distal ends of the tibia and fibula (Kear & Cooke 2001) and in the reduced amount of musculature they possess. Mentioned above as a possible weight-saving specialisation for their arboreal lifestyle, it means that tree-kangaroos are more similar in the general form of their musculature to climbing phalangers like Pseudocheirus than to normal kangaroos (Grand 1990).

A general characteristic of marsupials is the absence of a patella (with bandicoots being an annoying exception… the fact that bandicoots also have a placenta [yes, you read that right] means that some authors have actually seriously regarded bandicoots as placentals, not as marsupials). However, marsupials do have a fibrocartilaginous structure in the same place, generally termed a patelloid. Macropods mostly have what’s been termed a ‘type IV’ patelloid, but tree-kangaroos differ in having a proportionally much smaller, ‘type III’ patelloid that’s elsewhere seen in the potoroos (Potoroidae) (Reese et al. 2001). Potoroos are small macropods, thought to have diverged during the Late Oligocene – between 30 and 25 million years ago – from the rest of the macropod radition (Kear et al. 2008, Prideaux et al. 2010). Whether they should or should not be included within Macropodidae remains the topic of debate.

These primitive character traits seen in tree-kangaroos – those phalanger-like limb proportions, the very un-kangaroo-like amount of musculature they possess, that small patelloid, and that small amount of tibia-fibula contact – have conventionally been interpreted as reversals. But could they show that tree-kangaroos are, in actuality, truly primitive with respect to macropodid macropods, and hence outside the clade that includes most or all other lineages within Macropodidae?

Exactly this was suggested by Paul Hopkinson of San Diego State University in 1991 (Hopkinson 1991). In a conference abstract titled ‘Systematic position of Dendrolagus (Marsupialia, Macropodidae) and its implications for the ancestral lifestyle of kangaroos’, he specifically stated “It is suggested that the present systematic position of Dendrolagus within the Macropodinae is incorrect and that Dendrolagus is the sister taxon of all other Macropodidae”.

I’ve been unable to find out whether Hopkinson’s abstract ever formed the basis for a technical paper (do say if you know otherwise), but it does seem that he submitted a 1994 thesis on the same topic (Hopskinson 1994). I haven’t been able to track this down. Anyway, reading between the lines, it seems very clear that Hopkinson was positing (1) a basal position within Macropodidae for Dendrolagus and the exclusion of Dendrolagus from the macropodid clade Macropodinae (sorry, you have to keep up with the names for this to make sense), (2) the presence of various primitive, phalanger-like morphological details in Dendrolagus, and (3) an arboreal ancestry for Macropodidae as a whole, with Dendrolagus serving as a model ancestor for the prototype macropod. If the title for this article has been lost on you, what I’m getting at is the idea that tree-kangaroos might be the ‘ancestral form’ for the terrestrial macropod radiation, in the same way that bird-like, climbing archosaurs are ancestral for dinosaurs in the non-standard (and non-parsimonious) ‘Birds Come First’ model.

Could Hopkinson have been right? Well, as usual with surprising ‘non-standard’ hypotheses… no, it doesn’t seem that the bulk of evidence supports the ‘Tree-Kangaroos Come First’ hypothesis. My apologies if I created the impression that a ‘basal position’ for tree-kangaroos is likely.

Those ‘primitive’ characters present in tree-kangaroos really must be interpreted as reversals since they’re substantially out-weighed by a more impressive list of other characters. These place tree-kangaroos within the macropodid clade Macropodinae (e.g., Kear et al. 2008, Prideaux et al. 2010), not “outside the group that includes true kangaroos and wallabies” as implied by traditional classifications (e.g,. Nowak 1998), and definitely not outside the rest of Macropodidae as per Hopkinson. Molecular data supports the nesting of tree-kangaroos within Macropodinae as well (e.g., Baverstock et al. 1989, Kirsch et al. 1995, Burk et al. 1998).

What I find particularly interesting is that both morphological and molecular phylogenies have frequently (though not ubiquitously) found tree-kangaroos to be especially close to rock-wallabies (Petrogale). Rock-wallabies are small, nocturnal, group-living macropods that occur throughout much of Australia but aren’t found on New Guinea. Most recently, Prideaux et al. (2010) used the name Dendrolagini for the Dendrogale + Petrogale clade, and recovered it as the sister-taxon to Macropodini, the clade within Macropodinae that includes ‘true’ kangaroos and wallabies (again, I hope you’re following the fact that macropod, macropodid and macropodine all have separate meanings and are not interchangeable).

Charles De Vis actually suggested as long ago as 1887 that tree-kangaroos might be close kin of rock-wallabies since he perceived a strong superficial similarity between the two, and others commented on the same possibility in later years. Rock-wallabies make very logical close relatives of tree-kangaroos given that they’re agile climbers, well able to hop about on rocks, cliffs and on sloping trees and low branches*. Rock-wallabies also recall tree-kangaroos in often being fairly brightly coloured, boldly patterned, and in possessing long, cylindrical, sometimes slightly bushy tails.

* Martin (2005) described how people who live alongside coastal populations of the Proserpine rock-wallaby P. persephone in northern Queensland actually believe these animals to be tree-kangaroos since they often see them climbing in trees.

Note also that the ‘Tree-Kangaroos Come First’ hypothesis would require that tree-kangaroos be pretty old, since both fossils and molecular data show that the macropod radiation was well underway by the Late Oligocene. However, all fossil tree-kangaroos are from the Pleistocene or perhaps the Late Pliocene. Furthermore, some of these fossil forms share skeletal characters with Petrogale (Prideux & Warburton 2008). And molecular clock estimates have led some authors to suggest that tree-kangaroos originated sometime within the last 8 million years (Campeau-Péloquin et al. 2001), in which case they aren’t old at all but actually a fairly recent invention.

Is the idea that tree-kangaroos are ‘primitive’, morphologically and ecologically archaic macropods any more appealing than the idea that they’re actually highly specialised arboreal members of an otherwise terrestrial radiation? Frankly, who cares: we should be all about following the evidence, and the data putting tree-kangaroos deep within Macropodidae, and specifically within Macropodinae and close to rock-wallabies, is actually very good.

In fact, I personally think this idea is more interesting than Hopkinson’s alternative proposal. It means that a highly specialised, morphologically aberrant lineage is placed smack-bang in the middle of all those mostly terrestrial macropodine kangaroos. Would we ever think that macropodines, of all marsupials, would take to the trees, to learn how to drop safely from heights of over 20 m?

For previous Tet Zoo articles on marsupials and other metatherians (note that they’re horribly under-represented at Tet Zoo), see...

Hopkinson, P. 1991. Systematic position of Dendrolagus (Marsupialia, Macropodidae) and its implications for the ancestral lifestyle of kangaroos. Journal of Vertebrate Paleontology 11 (supp. 3), 36.

Hopkinson, P. J. 1994. The Systematic Position of Tree-Kangaroos (Dendrolagus: Macropodidae: Marsupialia) and its Implications for the Ancestral Lifestyle of Kangaroos. PhD Thesis, San Diego State University.

Kear, B. P. & Cooke, B. N. 2001. A review of macropodoid systematics with the inclusion of a new family. Memoirs of the Association of Australasian Palaeontologists 25, 83-101.

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

ABOUT THE AUTHOR(S)

Darren Naish

Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com!

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